1. Field of the Invention
The present invention relates to a liquid crystal display device suitable for an achromatic black and white (b/w) display.
2. Description of the Related Art
Conventional liquid crystal display devices can be classified into, e.g., a TN type, a DS (dynamic scattering) type, a GH type, a DAP (deformation of aligned phases) type, and a thermal write type in accordance with their operation modes. Of these devices, the TN liquid crystal display device has been often used as a display device for a portable calculator, a measuring instrument, and the like.
In recent years, a higher information content and a larger display area have been required for a display device for use in a wordprocessor, a personal computer, and the like. However, the conventional TN liquid crystal display device has problems of, e.g., an insufficient contrast and a narrow viewing angle. In this case, the viewing angle is taken to mean an angle at which a display can be clearly seen. For this reason, the TN liquid crystal display device cannot satisfy the above requirements. Therefore, a demand has arisen for a liquid crystal device having a new operation mode satisfying the above requirements.
As a liquid crystal display device capable of satisfying these requirements, Japanese Patent Disclosure (Kokai) No. 60-10702 discloses an electrically controlled birefringence, supertwisted birefringence effect (SBE) liquid crystal display device. This SBE liquid crystal display device comprises a cell obtained by sealing two transparent substrates arranged to oppose each other by a distance of 3 to 12 .mu.m. Transparent electrodes are formed on at least opposing surfaces of the two transparent substrates, respectively. A nematic liquid crystal is filled in the cell. Examples of the nematic liquid crystal for use in the SBE liquid crystal cell are cyclohexane-, ester-, biphenyl-, and pyrimidine-based liquid crystals. A chiral agent is added to the nematic liquid crystal so that the molecular axes of the liquid crystal molecules are rotated through an angle of 180.degree. to 360.degree. between the pair of substrates. In addition, since an alignment film is formed on the surface in contact with a liquid crystal of the substrate, the molecular axis of the liquid crystal has a tilt angle (.theta.) larger than 5.degree. with respect to the substrate surface. Retardation R of this liquid crystal cell is given by: EQU R=.DELTA.n.d.cos.sup.2 .theta.
The value of the retardation R is 0.6 to 1.4 .mu.m. In the above equation, .DELTA.n represents the optical anisotropy of a liquid crystal composition in the liquid crystal cell, d is the cell thickness (substrate spacing), and .theta. is the tilt angle.
Of the SBE liquid crystal display devices, a device in which the molecule axes of the liquid crystal are rotated through 270.degree. is well known. In this device, polarizers are preferably arranged in front and rear of a liquid crystal cell. Most preferably, a transmission axis of the front polarizer arranged in front of the liquid crystal cell has an angle of about 30.degree. clockwise with respect to an alignment direction of liquid crystal molecules on the front substrate of the liquid crystal cell, and a transmission axis of the rear polarizer arranged in rear of the liquid crystal cell has an angle of about 30.degree. counterclockwise or about 60.degree. clockwise with respect to an alignment direction of liquid crystal molecules on the rear substrate. When the angle between the transmission axis of the rear polarizer and the molecular axis of the liquid crystal molecule on the rear substrate is about 30.degree. counterclockwise, bright yellow is displayed in a nonselect state, and black is displayed in a select state (yellow mode). When the angle is about 60.degree. clockwise, deep blue is displayed in the nonselect state, and white is displayed in the select state (blue mode).
In the SBE liquid crystal display device, light transmission largely changes in accordance with even a small change in voltage applied to the device. Therefore, according to the SBE liquid crystal display device, even in case of a display with a large line number operated by a multiplexing drive, a high contrast and a wide viewing angle can be obtained.
SID' 86 DIGET, P. 122 describes a supertwisted (ST) liquid crystal display device in which a tilt angle is decreased and a twist angle of liquid crystal molecule is 100.degree. to 200.degree.. A term "twist angle" means an angle between a molecular axis of a liquid crystal molecule on one substrate and that on the other substrate.
Japanese Patent Disclosure (Kokai) No. 60-73525 discloses a liquid crystal display device using a liquid crystal cell in which retardation R is 0.5 to 0.8 .mu.m and a twist angle of liquid crystal molecule is 270.degree.. In this liquid crystal display device, two polarizers are arranged to sandwich the liquid crystal cell. According to this patent disclosure, it is preferred that an angle defined between optical axes of the two polarizers is substantially 90.degree..
A basic arrangement of the SBE or ST liquid crystal display device is shown in FIG. 1A. As shown in FIG. 1A, the SBE or ST liquid crystal display device has a liquid crystal cell 5, and two polarizers 3 and 4 arranged to sandwich the liquid crystal cell 5. The cell 5 comprises opposing substrates 1 and 1', and a liquid crystal composition held between the substrates. A polarization state of light transmitting through the device shown in FIG. 1A is shown in FIG. 1B. Light emitted from a light source is transmitted through the polarizer 3 to become linearly polarized light 103. The light 10 is transmitted through the cell 5 to become elliptically polarized light 101'. The shape of this ellipse is determined by a twist angle .PSI. of a liquid crystal of the cell 5, retardation R, and a wavelength .lambda. of the light. A part of the light 101' transmitted through the cell 5 is transmitted through the polarizer 4 and sensed by a human eye.
As described above, the transmission of the light in the above liquid crystal display device depends on the shape of the ellipse of the elliptically polarized light produced upon transmission through the liquid crystal cell. And the shape of the ellipse of the elliptically polarized light depends on the wavelength of the light. Therefore, the transmitted light through the device generally becomes chromatic. For example, as described above, the SBE liquid crystal display device exhibits yellow (yellow mode) or blue (blue mode) color in the nonselect state. This property is sometimes effective in special applications. However, the property is undesirable when these liquid crystal display devices are used as display units for a personal computer and office equipment. In addition, when the display surface is chromatic, its readability is evaluated to be inferior to that of a b/w display.
The TN liquid crystal display device can be used as a color display unit by arranging a color filter on a substrate. In the above liquid crystal display device, however, since the display surface becomes chromatic, its color reproducibility degrades.
In addition, the color of the above liquid crystal display device largely changes in accordance with a viewing angle or a temperature.
As a device which solves the above problems, Appl. Phys. Lett. 50(5), 1987, P. 236 describes an OMI liquid crystal display device. In this OMI liquid crystal display device, a twist angle of liquid crystal molecule is 180.degree. and the value of retardation R is 0.5 to 0.6 .mu.m. This device has two polarizers arranged such that a transmission axis of one of the polarizers is parallel to a rubbing axis of a substrate and an angle defined between absorbing axes of the two polarizers is 90.degree..
In the OMI liquid crystal display device, however, a change in transmission of the liquid crystal is not so large a compared with a change in voltage to be applied to the liquid crystal. Therefore, when a drive duty ratio is decreased, problems such as an insufficient contrast radio, a narrow viewing angle, and a dark background arise.
Double-cell ST (D-ST) liquid crystal display devices disclosed in Japanese Patent Disclosure (Kokai) Nos. 57-46227, 57-96315, and 57-125919 can solve the problems of the OMI liquid crystal display device, especially, the problems of a dark background and insufficient contrast. As shown in FIG. 2A, each ST liquid crystal display device comprises two adjacent liquid crystal cells 5 and 6, and two polarizers 3 and 4 arranged before and after the cells 5 and 6, respectively. The cells 5 and 6 have substantially the same retardation values and absolute value of twist angles of liquid crystal molecule in the cells. The cells 5 and 6 are arranged such that twist directions of liquid crystal molecule in the cells 5 and 6 are opposite to one another. A term "twist direction" means a direction along which the molecular axes of liquid crystal spirally arranged.
A polarization state of light through the liquid crystal display device having the above arrangement is shown in FIG. 2B. Light emitted from a light source is transmitted through the first polarizer to become linearly polarized light 103. The light 103 is then transmitted through the first liquid crystal cell 5 to become elliptically polarized light 101'. The light 101' is transmitted through the second liquid crystal cell 6 to become linearly polarized light 102'. This is because the cell 6 has substantially the same retardation and twist angle as those of the cell 5 and has the twist direction of liquid crystal molecule opposite to that of the cell 5. The light 102' transmitted through the cell 6 is transmitted through or absorbed by the second polarizer 4 in accordance with the direction of a vibration surface.
In this D-ST liquid crystal display device, it is important that the first and second liquid crystal cells 5 and 6 optically complement each other. For this purpose, in particular, the retardation values of the cells must be substantially equal to each other. A difference between the retardation values of the two liquid crystal cells is, e.g., .+-.0.05 .mu.m.
In this D-ST liquid crystal display device, electrodes are formed on substrates 1 and 1' of the cell 5 and driven as in a normal dot-matrix liquid crystal display device. The cell 6 is, however, used to simply compensate the elliptically polarized light produced by the cell 5 and therefore need not be driven. Therefore, it is not necessary an electrode is formed on substrates 2 and 2'.
The D-ST liquid crystal display device can provide an achromatic b/w display. This means that all the incident light components can be effectively used. Therefore, since a light amount on the display surface is large, a bright screen can be obtained. In addition, the number of scanning lines can be increased in this D-ST liquid crystal display device.
This D-ST liquid crystal display device uses two liquid crystal cells requiring high manufacturing precision. Therefore, a manufacturing cost is increased to result in an expensive device.
Furthermore the viewing angle of this D-ST liquid crystal display device, however, is narrower than those of the SBE and OMI liquid crystal display devices.